The present invention relates to the general field of heat-transfer devices. More particularly, the present invention is directed to a freeze-tolerant condenser for a closed-loop heat transfer system.
Waste heat is generated aboard all types of spacecraft by propulsion systems, electrical systems, on-board experiments, human metabolic processes and the like. To avoid overheating of these sources, the waste heat must be conveyed away from the heat source to a heat sink located either within the spacecraft, e.g., to heat a region of the spacecraft that does not have its own heat source, or outside the spacecraft to rid the spacecraft entirely of the waste heat.
One type of system used for transporting heat from a waste heat source to a heat sink is a capillary pump loop (CPL) system, which is a highly-efficient system requiring little or no external power. A CPL system is a two-phase system that utilizes a vaporizable liquid, or working fluid. Ammonia is the most desirable working fluid because it has a very good heat-transfer ability. Heat is absorbed by the working fluid when it is changed from a liquid phase to a vapor phase upon evaporation, and heat is released from the working fluid when condensation of the vapor occurs. The CPL system includes a heat pipe containing a capillary structure, such as a porous wick, and a continuous loop that provides a vapor phase flow region, a condenser region and a liquid return region.
Referring now to FIG. 1, there is shown a conventional condenser assembly 10 used in a CPL system. The condenser assembly comprises a condenser tube 12 and a radiator panel 14, or heat sink, in thermal communication with condenser tube 12. Condenser tube 12 has an inlet 16 and an outlet 18. When condenser assembly 10 is part of a closed loop, two-phase heat-transfer system, inlet 16 is in fluid communication with an outlet of an evaporator (not shown) and outlet 18 is in fluid communication with an inlet of the evaporator. The evaporator collects waste heat from one or more heat sources aboard the spacecraft in a manner known in the art.
During normal operation, the closed-loop system is charged with a working fluid, such as ammonia. The working fluid is in its liquid phase when it enters the evaporator. As the working fluid moves through the evaporator, waste heat collected by the evaporator is transferred to the working fluid, thereby changing the working fluid from its liquid state to its vapor state. The working fluid vapor then exits the evaporator and enters condenser assembly 10 through inlet 16 of condenser tube 12. As the working fluid flows through condenser tube 12, it gives up heat to radiator panel 14 and condenses into its liquid phase. The condensate then exits from outlet 18 and returns to the evaporator where it is heated and vaporized by the waste heat. At the design heat load, entire radiator panel 14 is nearly isothermal, i.e., the temperature is uniform along the entire length and across the entire width of radiator panel 14.
A problem with conventional condenser assemblies arises when the heat load is less than the design heat load. Because the boundary conditions for the condenser tube inlet temperature of the working fluid and the heat sink temperature remain the same even when the heat load is reduced, radiator panel 14 rejects more heat than is carried by the working fluid flowing into condenser assembly 10. When this happens, the working fluid exits condenser assembly 10 in a highly sub-cooled state. Sub-cooling can drastically reduce the efficiency of the CPL system because there may not be enough waste heat to overcome the sub-cooling to allow the liquid working fluid to evaporate. Since it is desirable to have only a small sub-cooling for a near-isothermal boundary condition on the radiator panel, the amount of sub-cooling must be reduced.
Currently, there are two approaches to eliminating large sub-cooling. The first approach is to add heat to the system, either directly to radiator panel 14 or to the working fluid before it reaches the evaporator. A problem with this approach is that the extra power needed to supply the additional heat is usually not available due to the need to keep power generating systems as small and lightweight as possible. The second approach is to bypass some of the vapor exiting the evaporator directly to the liquid exiting the condenser assembly 14 to pre-heat the liquid before it returns to the evaporator. A problem with this approach is that it requires active mechanical controls that are rife with possibilities for mechanical failures that cannot be tolerated in spacecraft applications. In addition, this approach requires that the vapor phase be introduced into the liquid phase, which may cause flow instabilities or even destructive condensation-induced waterhammer events.
Another problem with conventional condenser assembly 14 arises when the effective heat sink temperature of the radiator panel falls below the freezing point of the working fluid, which, as noted above, is typically ammonia. In such case, the working fluid can freeze at heat loads less than the design heat load. The frozen working fluid forms a plug in condenser tube 12 that shuts down the entire CPL system by preventing the remaining unfrozen working fluid from flowing and transporting waste heat away from the evaporator. The present remedy for this situation is to provide a supplementary heater and control system therefor to radiator panel 14, but this adds unnecessary weight. Another approach is to select a different working fluid having a lower freezing point. However, all of the known fluids that do not freeze at low operating temperatures have very low heat-transfer ability in comparison to ammonia.
The present invention is directed to an artery for heat-transfer device. The artery comprises a side wall having an outer surface, an inner surface and a plurality of capillaries formed therein. Each capillary extends from the outer surface to the inner surface along a longitudinal axis and has a side wall including a surface having formations thereon formed by melting and subsequent re-solidification of the side wall of the capillary. Each capillary also is linear, has a substantially uniform cross-sectional shape transverse to the longitudinal axis, and has at least one dimension transverse to the longitudinal axis that is no greater than 50 microns.
In another aspect, the present invention is directed to a two-phase heat-transfer system utilizing a working fluid. The system comprises a condenser that includes an enclosure having an interior surface and a length. The enclosure contains a first interior space and a second interior space. The enclosure is provided for containing the working fluid. A partition is located within and extends along at least a portion of the length of the enclosure. The partition separates the first interior space from the second interior space and has a plurality of capillaries formed therein. Each capillary extends from the outer surface to the inner surface along a longitudinal axis and has a side wall including a surface having formations thereon formed by melting and subsequent re-solidification of the side wall. Each capillary also is linear, has a substantially uniform cross-sectional shape transverse to the longitudinal axis, and has at least one dimension transverse to the longitudinal axis that is no greater than 50 microns.
In yet another aspect, the present invention is directed to a heat-transfer system utilizing a working fluid. The system comprises a condenser assembly that includes a plurality of condensers, a heat sink, a first header and a second header. Each condenser includes an enclosure having an interior surface and a length. The enclosure contains a first interior space and a second interior space. The enclosure is provided for containing the working fluid. A partition is located within and extends along at least a portion of the length of the enclosure. The partition separates the first interior space from the second interior space and has a plurality of capillaries formed therein. The heat sink is in thermal communication with the enclosure of each condenser. The first header is in fluid communication with the first interior space of each of the condensers, and the second header is in fluid communication with the second interior space of each of the condensers.
The present invention is also directed to a method of forming an artery for a two-phase heat-transfer system. The method comprises the steps of providing a solid-walled body having an outer surface and an inner surface and laser-micromachining a plurality of capillaries into the body. Each capillary extends from the outer surface to the inner surface along a longitudinal axis and has at least one cross-sectional dimension transverse to the longitudinal axis of less than 50 microns.